1. Field of the Invention
The present invention relates generally to microfluidic components for diagnostic kits and, more particularly, to methods and devices for manipulation and characterization of particles with alternating current (AC) electric fields.
2. Related Art
Dielectrophoretic analysis and separation of particles and bioparticles such as cells, viruses, proteins and DNA using alternating current (AC) and direct current (DC) electric fields are potentially powerful microfluidic technologies that can be used in medical and environmental diagnostic kits and high-throughput drug screening. Recent development efforts have tried to exploit dielectrophoresis for particle transportation, separation, focusing, characterization and release. For medical and environmental diagnostic kits, the goal would be to rapidly concentrate, identify and determine the viability of pathogens in dilute samples with less than one thousand bioparticles per cc.
A plethora of approaches with disjointed electrode designs have not been able to employ dielectrophoresis with the necessary speed to attain the rapid processing time that chip-based diagnostics require. The greatest challenge is that the velocity imparted on a particle via dielectrophoresis scales as the second power of both the particle radius and the electric field, meaning that high electric fields are necessary for rapid particle manipulation. Unfortunately, even with micro-fabricated electrodes the field is typically less than 100 V/cm. This is because with conventional inter-digitated and disjointed electrode designs the electrode RMS voltage cannot exceed 5V due to Faradaic electrochemical reactions that contaminate samples and lead to bubble generation. Consequently, with disjointed electrodes a typical velocity imparted on a particle via dielectrophoresis is less than 10 microns per second. Therefore, manipulation and characterization must be carried out in extremely small channels (<100 microns) in order to be completed in a reasonable time frame.
The physical limitations of using disjointed electrodes results in long processing times for typical sample sizes and can ultimately lead to errant measurements despite the long wait. The slow dielectrophoretic motion requires the use of confined geometries or waiting tens of minutes to hours for processing a larger volume. The challenges associated with characterizing particles with disjointed electrodes arise from electro-osmotic flow that often occurs near the electrode surfaces. If the device is to measure a property of the particle based upon its dielectrophoretic motion, electro-osmotic flow could camouflage the behavior of the particle that is driven by dielectrophoretic motion alone. Regardless of the chosen application, the key problem that arises is that the throughput for processing steps that make use of dielectrophoresis is limited to the range of picoliters to nanoliters per second when disjointed electrodes are employed. This throughput is inadequate for rapid diagnostic applications of dielectrophoresis, which require rapid (<1 minute) particle manipulation and analysis of realistic sample volumes that range from 0.1 to 1.0 milliliters.